The present invention relates to an optical deflector provided with a scanning mirror rotatable about a shaft by pneumatic pressure. This sort of mirror forms a part of a scanner system adaptable for an image or information processing device and a measurement apparatus. One prior art scanning mirror rotatable about a shaft by pneumatic pressure will be explained hereinafter with reference to the attached drawings, including FIG. 8, FIG. 9, FIG. 10 and FIG. 11.
FIG. 8 indicates a sectional view of a scanning mirror 5 rotatably supported by a vertical shaft la accommodated in a cavity V of a motor case 3 having a cover 4. The lower end of the shaft is fixedly held by the motor case. The shaft 1a is provided with a pair of herringbone grooves Ha and Hb formed around the periphery of the shaft 1a, as shown in FIG. 9. the lower end of the shaft is shrinkage fitted to the motor case 3 while the upper end thereof is fitted to the cover 4. A cylindrical sleeve 2 is rotatably arranged about the shaft 1a, leaving a clearance therebetween. A cylindrical magnet 22 is fixedly disposed around the lower portion of the sleeve 2. On the vertical inner wall of the motor case 3, a yoke 16 having coil 15 mounted thereon is fixed. Between the cover 4 and the motor case 3, there exists an O-ring 21, while another O-ring 20 is located between the shaft la and the cover 4, whereby the cover 4 and the motor case 3 are hermetically sealed. Numeral 17 is a Hall element holder to which a Hall element 18 is fitted. A thrust magnet 12 located on an inner base of the motor case 3 is arranged around the shaft 1a. Numeral 13 is another magnet, facing the thrust magnet 12 and having an opposite polarity, while numeral 14 is a spacer located underneath the magnet 22 and fitted to the magnet 13.
A hub 6 fixed around the sleeve 2 is provided with a polygon mirror 5, which is disposed horizontally on the hub 6 by means of a screw 9. Thus the sleeve 2, the hub 5 and the polygon mirror 5 are simultaneously rotatably arranged about the shaft 1a. A pair of magnets 10 and 11, each having an opposite pole N and S, are disposed face-to-face on the top end of the shaft 1a and on the under surface of the cover 4, respectively. It should be noted that instead of a polygon mirror 5, any type of flat mirror may also be adaptable.
With the energization of the magnet 22 by electrical current supplied via the coil 15, a motor driving means which, in this embodiment, is composed of a magnet 22 and the sleeve 2, begins to rotate about the shaft 1a with the sleeve 2 being supported by the spacer 14 and the magnet 13. Simultaneously with the rotation of the sleeves, a mirror rotating means which, in this case, is composed of the hub 6, the magnet 22 and the sleeve 2, is rotated about the shaft 1a accompanied by the mirror 5.
When the sleeve 2 is thus rotated, a clearance is maintained between the lower surface of the case 4 and the upper end of the sleeve 2 by means of a repulsion force created by the pair of magnets 10 and 11 and the pair of magnets 12 and 13, respectively positioned face-to-face and provided with opposite poles.
Corresponding to the rotation of the mirror rotating means and the motor driving means, as explained heretofore, air is sucked from suction intakes s, as show in FIG. 10, into the clearance existing between the shaft 1a and the sleeve 2 (FIG. 10), and a dynamic pneumatic pressure is created therein with the aid of the herringbone grooves Ha and Hb, whereby a constant rigidity of the shaft 1a is maintained.
FIG. 11 is a chart of the pneumatic pressure distribution existing in the clearance between the sleeve 2 and the shaft 1a when the sleeve 2 is rotated accompanied by the mirror 5. FIG. 10 indicates a sectional view of the sleeve 2 and a flat view of the shaft 1a. The structure shown in FIG. 10 may hereinafter be called a dynamic pneumatic pressure creation means wherein, corresponding to the rotation of the sleeve 2 as explained, air is sucked via suction intakes s into the dynamic pneumatic pressure creation means, with the result that pneumatic pressure inside the dynamic pneumatic pressure creation means is maintained higher than the circumferential air pressure outside thereof. Pneumatic pressure around the outside of the dynamic pneumatic pressure creation means and in the cavity may be maintained equal to atmospheric pressure.
The air pressure distribution inside the dynamic pneumatic creation means shown in FIG. 10 is illustrated in FIG. 11. In FIG. 11, the ordinate indicates dimensionless air pressure P, which represents an atmospheric pressure, while Z is the abscissa. The atmospheric pressure around the outside of both ends of the dynamic pneumatic pressure creation means is indicated by P=1, while on the inside of the means the pressure is indicated by P&gt;1. Therefore, the outside or surrounding air pressure of the dynamic pneumatic creation means is equal to atmospheric pressure.
Windage loss W due to the rotation of the mirror is usually indicated as follows:
W=Ps.times.Nr.sup.3 .times.Km, where PA1 Wo=1.0.times.Nr.sup.3 .times.Km (watt).
Ps: circumferential pressure, PA2 Nr: number of rotation, and PA2 Km: coefficient of mirror configuration.
In this prior art, windage loss Wo may be indicated as follows:
As is explained heretofore, windage loss W due to the rotation of the mirror is usually shown as: W=Ps.times.Nr.sup.3 .times.Km (watt), while in the prior art, as the circumferential air pressure around the pneumatic pressure creation means and in the cavity is already set as Ps=1, and the windage loss may be indicated as Wo=Nr.sup.3 .times.Km (Watt). Therefore it is impossible to reduce the windage loss less than the value shown above.
It is natural in the case of the prior art that in proportion to the increase of the number of rotations Nr, or due to the coefficient of the configuration of the mirror, the windage loss becomes larger. If the motor torque becomes smaller than the windage loss, a motor of a large size, which requires the supply of more electric current, is required to resolve the situation, which results in bringing forth the generation of more heat by the increasing supply of electric current. In order to prevent the generation of heat, a type of radiation system must be installed, with a resultant increase in the production costs, coupled with the difficulty of the miniaturization of the device at a low cost.